Process for forming cross-linked polyethylene film



1965 c. J. BENNING ETAL 3,201,503-

PROCESS FOR FORMING CROSS-LINKED POLYE'IHYLENE FILM Filed Jan. 31, 1962l m u:

m N f A I N OGR NOM. NI-GR T WM? T NE W A IBSI J N K wmm LA @RF UnitedStates Patent 3,201,503 FRQQESS FQR .FQRMTNG CRS=LINKED PQLYETHYLENEFILM Calvin J. Banning, Clarlrsville, Razmic S. Gregorian, Silverfipring, and Frank X. Werher, Rockville, Md, assignors to W. R. Grace &(10., New York, N.Y., a corporation of Connecticut Filed Jan. 31, 1962,Ser. No. 170,214 21 Gaines. (Cl. 264-495) This invention is directed toa continuous process for making shrinkable polyethylene film.

In summary, the process comprises the following steps:

(1) Forming a substantially homogeneous mixture of a normally solidpolyethylene with a cross-linking agent, preferably an organic peroxide;

(2) Shaping the mixture into a tube by extruding at temperaturessufficient to melt the polyethylene but below the gel point of themixture;

(3) Feeding the tube as it leaves the extruder into a curing zonemaintained at temperatures suflicient to heat the tube at least to thedecomposition temperature of the crosslinking agent and preferablyprovided with an inert atmosphere;

(4) Passing the tube through said zone at a rate sufficient to decomposeat least about 85% by weight of the crosslinking agent, whereby the tubeis cured, i.e., the polyethylene is crosslinked;

(5) Expanding the crosslinked polyethylene tube by the trapped bubbletechnique and recovering the film produced.

The resultant film can then be used for the standard uses ofpolyethylene film. Its high shrink energy, strength, and clarity make itespecially suitable for shrink-fit packaging of numerous articles, e.g.,foods. That is, it can be used to wrap hams, chickens, etc., followed byevacuation of the air between the film and the wrapped object, followedfinally by heating the wrapped object to shrink the film closely againstthe object.

One embodiment of the invention is described in the following example.

EXAMPLE 1 The polyethylene used was a commercially available low density(0.92) branched type made by the wellknown high pressure process. Theperoxide used was 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane of theformula:

A solution of 30 grams of the peroxide in 200 cubic centimeters ofpetroleum ether was sprayed on pellets of the polyethylene in atwin-shell blender, so as to coat the pellets with about 0.75 weightpercent peroxide.

The thus prepared homogeneous polyethylene-peroxide mixture was thencharged to a tube extruder. The tube extruder can be of any conventionaldesign for the extrusion of polyethylene tubing. In this particularapparatus the material was screw-fed through a 1" diameter heatedbarrel, 20 inches long, to a die face for one inch diameter tubing, 20mils wall thickness. The extruder was maintainedat temperatures of 230to 300 Fahrenheit (230 at the feed hopper to 300 at the die), and wasoperated so as to produce between 2 /2 and 6- /2 pounds of tubing perhour. Under these conditions premature crosslinking in the extruderbarrel or die was held to a minimum (less than 2% gel formation).

The extruded tube was cured (crosslinked) by passing it upward through afurnace. The furnace interior can be several times the diameterof thetube, and 10 to 12 ice times as long. In this particular run the furnaceinterior was 3 inches inner diameter and 12 inches long. Heating was byelectrical resistance wire. The temperature inside the furnace was about500 Fahrenheit. Residence time in the furnace was about 40 seconds. Theinterior of the furnace was continually flushed with pro-heated nitrogento minimize oxidative degradation. These conditions give a gel contentof the cured tube of about 60%, as determined by extraction of a samplein boiling toluene.

The temperature of the furnace is important. The temperature should besuificiently high to provide rapid crosslinking, and yet not so high asto scorch the tube. Rapid crosslinking is essential, as the tube doesnot become self-supporting until it is crosslinked. For instance, underthe conditions described in this example, the tube was only about /3crosslinked (20% gel) at a furnace temperature of 300 Fahrenheit and wasvery difficult to support on account of its low melt viscosity. Even at350 Fahrenheit there was still a slight flow. At 400 Fahrenheit the tubebecame self-supporting, and approached maximum strength between 450 and550 Fahrenheit. Above 550 there was evidence of scorch ing at normalfurnace residence times.

The cured tube can be cooled and coiled up fiat as tape, or it can beexpanded immediately in the next operation while still near its meltingpoint.

To expand the tube, the trapped bubble technique is used. This techniqueis well-known, and is described, for example, in Kirk-Othmer,Encyclopedia of Chemical Technology, vol. 14, pp. 751-752. As applied tothe instant operation a 3 foot length of tube (at approximately themelting point of the polyethylene, i.e., centigrade) is drawn up andpinched oif between a first and a second set of rolls. This sectioncontains nitrogen, air, or other inert gas under pressure. The gas canbe added by syringe, and the syringe hole sealed with cellulose tape. Asthe tube is now advanced to the second set of rolls, the gas pressureexpands the advancing tube section into a bubble, which remains the sameshape as the tube advances continuously around it. Bubble expansionprovides biaxial orientation of the resultant film. The ratio of bubblediameter to tube diameter can vary within wide limits, depending on thefilm thickness desired and the degree of stretching desired. A typicalratio is 5:1. Thus for a 1 inch diameter tube, the bubble diameter isgenerally about 5 inches and transverse stretching has a factor of five,so that (with no longitudinal stretching) a 20 mil tube is stretched toa film 20+5 or 4 mils thick. If an equal amount of longitudinalstretching is desired, the take up rolls on the far side of the bubbleare run five times faster than the tube extrusion rate, so that theultimate biaxially oriented film has a thickness of 20+5+5, or 0.8 mil.

The tubular film so formed is either rolled up as made, or is cut at theedges and rolled up as flat film on two separate rolls.

BiaxiaJly-oriented peroxide-crosslinked film prepared by the process ofthis invention is in general similar to oriented radiation-crosslinkedfilm. However, in respect of certain properties important in the fieldof food packaging, it is superior, as shown by the following comparativedata:

The homogeneous mixture of normally solid polyethylene resin andfree-radical generating crosslinking agent can be formed in any suitablemanner. One suitable method of blending comprises dissolving thecrosslinking agent in a solvent inert to the polyethylene (such aspetroleum ether), mixing the solution in appropriate amounts withpolyethylene resin pellets or ranules and gradually heating the mixtureto a temperature sutficient to evaporate the solvent but not so high asto decompose the crosslinking agent. In this method the resultantproduct comprises polyethylene pellets or granules having a uniformlydistributed surface coating of crosslinking agent. In another method theparticulate polyethylene resin and liquid or powedered crosslinkingagent are dryblended until the crosslinking agent is homogeneouslydistributed throughout the mass. As an example, polyethylene pellets(99.25% by Weight) and liquid 2,5- dimethyl-2,5-di(tertbutylperoxy)hexane were blended into a homogeneous mass after twenty minutes at roomtemperature in a Patterson-Kelley twin shell blender. Resin andcrosslinking agent can be very thoroughly mixed by fiuxing the mixtureon a two-roll mill or in a Banbury mixer at temperatures below the gelpoint of the admixture, if desired. This latter mode of mixing does notafford any significantly greater degree of homogeneity than thatobtained by the dry-blending or solventblending methods previouslydescribed and thus will ordinarily be uneconomical.

In the process of this invention the crosslinking agent is used in anamount of from about 0.2 to about 4.0 percent by weight, based on theweight of the polyethylene resin. Preferred amounts of crosslinkingagent are from about 0.5 to about 1.0 percent by weight, based on theweight of the polyethylene. Concentrations of crosslinking agent in theoperable range have little or no detectable effect on extruder outputrate when compared to polyethylene having no additive. The preferredconcentration range gives the best combination of process economy andefficiency. It is also within the purview of this invention to add otherknown ingredients, e.g., pigments, dyes, fillers, stabilizers, etc., tothe homogeneous mixture of normally solid polyethylene and crosslinkingagent. In preparing shrinkable film for use in food packaging it isgenerally unnecessary and often undesirable (because of possibletoxicity or other physiological problems) to use additives other thanthe freeradical generating crosslinking agent.

In the second step of the process described herein the substantiallyhomogeneous mixture of polyethylene and crosslinking agent is shapedinto a tube by extrusion at temperatures sufiicient to melt thepolyethylene but below about the gel point of the mixture. Preferablythe tube extrusion is accomplished in any of the well-known rotatingscrew extruders.

The term gel point as used herein means those temperatures at whichthere is sufficient decomposition of the crosslinking agent to cause gelformation in the polyethylene. The gel point of any specific mixturedepends upon numerous factors including, e.g., the particularpolyethylene and crosslinking agent, the amount of crosslinking agent,and the half-life of the crosslinking agent at various shapingtemperatures. Since the gel point of any specific mixture is dependentupon so many factors it is best determined by empirical methods, i.e.,by extruding a small sample and observing or determining whether anygelled polyethylene particles have been formed. As previously noted, theshaping step of the instant process must be accomplished with theformation of no more than about 2% (by weight) of gel. Thus, the termgel point is to be construed to include temperatures up to those whichproduce this result. Means for minimizing or avoiding gel formationduring the shaping step include adjusting the temperature of theextruder barrel and die, adjusting the extruder output rate, varying theamount of crosslinking agent or changing the particular crosslinkingagent used. Preferably, extrusion is accomplished with minimum backpressure by eliminating screen packs and the breaker plate and by usingstreamlined dies. When operating the extruder in this manner, there islittle chance of any holdup in the high temperature zones or in thetubular die of the extruder which might cause premature crosslinking.

The tube comprising polyethylene with crosslinking agent substantiallyuniformly distributed therein is usually fed to the high-temperaturecuring zone by pulling upwards through the curing zone or gravityfeeding in a substantially vertical downward direction. Feedinghorizontally or in directions other than substantially vertical can beaccomplished by the use of specialized equipment which will notphysically mar or otherwise damage the hot extruded tube and which atthe same time provides means for preventing the tube from collapsingupon itself. Because of the inherent simplicity in gravity threading itis preferred to extrude the tube downwards through the heated zone. Indownward extrusion some necking of the tube may occur at or just belowthe extruder die tube before the tube is self supporting. However, thisminor drawback can be overcome by initiating curing of the tube as soonas possible after it leaves the extruder.

Curing of the extruded tube is accomplished by passing it through aheated curing zone; or a series of contiguous heated curing zonesmaintained at different temperatures. One very satisfactory heated zoneis a tubular furnace brought to curing temperatures by means of electricresistance heaters. Other suitable heating apparatus will be apparent topersons skilled in the art. In any case, it is highly desirable and thususually preferred to perform the curing step in an inert atmosphere inorder to minimize oxidative degradation of the polyethylene and/ orretardation of the crosslinking reaction. One method of providing aninert atmosphere is to continually flush the heated curing zone with aninert gas (such as nitrogen, helium, argon or the like) which containsminimum amounts (preferably less than 50 parts per million) of oxygenimpurity. The use of carbon dioxide as the inert flushing gas has beenfound to give especially interesting results in that it inducesformation of carbonyl groupings in the end product film thus increasingthe printability thereof.

One disadvantage of the use of carbon dioxide is that it inhibits to aslight degree the crosslinking action (as evidenced by lower gel contentof the cured tube). This drawback is, however, alleviated by usinghigher concentrations of crosslinking agent than are used, for example,when the inert flushing gas is nitrogen.

At least a portion of the heated curing Zone through which the extrudedtube is passed (in order to crosslink the polyethylene) is maintained attemperatures above that required to heat the polyethylene tubing to thedecomposition temperature of the crosslinking agent. As used herein theterm decomposition temperature means a temperature at which thecrosslinking agent has a half life of less than about 1.0 minute, andpreferably less than about 0.5 minute. There is no practical means fordetermining the exact temperature of the polyethylene tube withoutdamaging the same. Hence, the exact temperature to which the heatedcuring zone must be raised is determined by empirical methods.Sufficicnt guides for determining such temperatures are furnished by thespecific examples herein, and further adjustment to obtain optimumconditions will be immediately apparent to those skilled in the artafter a cursory study thereof.

The polyethylene tubing is passed through the heated curing zone at arate which provides a residence time (at the decomposition temperatures)at least equal to three (3) half-lives of the curing agent, i.e.,residence time is suflicient to decompose about or more of the crosslinking agent. The term residence time asused herein is determined byuse of the following equation:

Residence Time: L2

where L =length of the heated curing zone in which the tubing is abovethe decomposition temperature and L T =the rate (in length per unit oftime) at which polyethylene tube is withdrawn from the zone; and where Land L are expressed in the same units (e.g., meters, feet, inches,etc.). Required residence times are easily obtained by varying eitherthe length L or the rate at which the polyethylene tube is withdrawnfrom the heated zone, or both.

Curing of the polyethylene tube should be as rapid as possible so thatthe tube quickly becomes self-supporting (that is, so that it will notdistort) after it leaves the extruder. The amount of cure needed to givethis result cannot be precisely stated. For high molecular weightpolyethylenes curing to about gel is suificient. Up to gel might berequired in other cases. Temperatures and residence times necessary forrapidly achieving this degree of curing depend upon such variables asthe temperature of the tube as it leaves the extruder die (i.e., how farbelow the gel point the shaped mixture is), the specific crosslinkingagent or agents used, the amount of crosslinking agent(s), and so on. Inorder to minimize or preclude distortion of the tube it will usually bepreferred to feed the tube immediately as it leaves the extruder to thecuring zone, and adjusting the conditions prevailing in the initialportion of the curing zone (that portion into which the extruded tube isfirst introduced) to provide at least about 20% gel within about 5 to 10seconds or less. Curing can then be completed under the same conditionsor at a slower rate under more moderate conditions, as desired.

Cured polyethylene tubing produced in the manner described above hasfinal gel levels of greater than about 20% by Weight. Under optimumconditions percent gel ranges from about 50 to about 75% by weight.Percent gel as reported in the examples is determined by extraction of asample of tubing weighing about 0.5 gram in'a refluxing solution oftoluene or xylene containing 0.3% by weight of2,6-di(tert-butyl)-4-methylphenol for about 20 hours and then drying andweighing unextracted residue (gel). The Weight of unextracted residuedivided by the weight of the original sample is multiplied by 100 tocalculate percent gel. Results of tests run on samples taken at variousdepths in the tube cross-section have shown that percent gel does notvary more than about 2%, indicating that crosslinking occurs in auniform manner over the full cross-section of the tube.

Cured polyethylene tube issuing from the heated curing zone is cooled toa temperature below that at which it adheres to itself. The temperatureto which the cured tube must be cooled depends upon the particular type(i.e., high or low density) of polyethylene therein, the degree to whichthe polyethylene. has been crosslinked, and other factors. It has beenfound that cured tubes of low density (about .91 to .925) polyethyleneshould be cooled to below about 70 Centigrade before coiled or otherwisemechanically or manually handled. High density polyethylenes 0.95 to0.98) can be handled after cooling below about 100 centigrade. Aparticularly store-d for later use or may be expanded immediately aftercooling. In either case, the tube must be reheated to a temperature atwhich it will expand. These reheating temperatures depend primarily uponthe type of polyethylene used to make the tubing. For low densitypolyethylenes, the expansion temperatures are between about Centigradeand about 110 centigrade, preferably from about Centigrade and aboutCentigrade as is well known to the art. Expansion temperatures for curedhigh density polyethylene tubing are about 20 Centigrade to about 30Centigrade higher than those for low density polyethylene tubing. Asnoted above, specific details of the process and apparatus used in theexpansion of polyethylene tubing to form film are known in the art andhence they need not be repeated herein.

In the fully continuous process of this invention, the curedpolyethylene tubing is cooled as described above (preferably byquenching), immediately fed through a second heat exchange device (e.g.,an infrared heater, a heated bath of inert liquid such as water, oil,mineral oil or a furnace, oven etc.) where it is reheated to expansiontemperatures and then between two pairs of pinch ir lflls where it isexpanded by an inert gas to form tubular Any of the various well-knowntypes of polyethylene can be used in making film by the process ofthisinvention. Such polyethylenes include the branched low-density (i.e.,about .910 to about .925) material already described in Example I aboveas well as the medium density materials and the newer linear highdensity (about .950 to .980) materials made by the Ziegler process(TlClg-Al. alkyl catalyst) and the Phillips process (hexavalent chromiaon silica-alumina support). The linear polyethylenes have melting pointsin the range of l36 centigrade, and therefore require peroxides (orother freeradical generating crosslinking agents) that provide gelpoints higher than these temperatures.

Crosslinking agents do not ordinarily have a sharp ecomposition point,except possibly at very high temperatures. In the usual case, the agentrequires several minutes to decompose substantially quantitatively, andthe rate of decomposition at a given instant is generally proportionalto the amount of material. Consequently, the decomposition rate for agiven material at a given temperature can generally be determined by itshalf life at that temperature. The half-life of any free-radicalgenerating agent can be readily determined by one skilled in the art. Inthe case of peroxides, for example, the determination involved isdescribed in Doehnert et al., Evaluation of Organic Peroxides on thebasis of Half- Life Data, Ann. Tech. Management Conf., ReinforcedPlastics Div., Soc. Plastics Ind, Inc. 13, sect. l-B, 1-8 (1958); Chem.Abs. 53, 18534i (1959).

Free-radical generating crosslinking agents which can be used in theprocess of the instant invention include organic peroxygen compounds andazonitriles. Suitable organic peroxygen compounds are diacyl peroxides,such as benzoyl and lauroyl peroxides; dialkyl peroxides such as diethylperoxide, di(tert-butyl)hydroperoxide, diisopropyl peroxide or the like;hydroperoxides, such as tert-butyl hydroperoxide or the like; peracids,such as acetoperacid, benzoperacid, succinic monoperacid and the like;peresters, such as ethyl perbenzoate, butyl perbenzoate and the like;and diaralkyl peroxides such as dibenzyl peroxide, dicumyl peroxide andthe like. Suitable azonitriles include dimethyl-oc,ot'-azodiisobutyrate,azodicyclohexane carbonitrile and other like compounds. Specificexamples of suitable free radical generating crosslinking compounds andtheir half lives are:

Crosslinking agent: Half-life Di(tert-butyl) peroxide 1 minute at C.Tert-butyl hydroperoxide 1 minute at 230 C. Dichlorobenzyl peroxide 1minute at 112 C. Tert-butyl peracetate 0.5 minute at 178 C.

Dicumyl peroxide 0.6 minute at 182 C.

7 Crosslinking agent: Half-1ife Diethyl peroxide 1 minute at 198 C.Di(tert-amyl) peroxide 1 minute at 182 C. Cyclohexyl peroxide 0.5 minuteat 226 C.

2,5-dimethyl-2,5-di-(tert-butylperoxy) hexane2,5-dimethyl-2,5-di-(tert-butylperoxy)-3-hexyne a Azobis( 01,7,7trimethylvalero-nitrile) a t Azobis(a cyclopropyl- 0.6 minute at 185 C.

0.6 minute at 192 C.

2 minutes at 91 C.

propionitrile) 1 minute at 118 C. Dimethyl a azodiisobutyrate 2 minutesat 138 C.

a,a-Azodiisobutyronitrile 2 minutes at 130 C.

A z dicyclohexane carbonitrile 2 minutes at 166 C. fl-Hydroxyethylazoa,'y dimethylvalero-nitrile 2 minutes at 182 C.

Especially preferred crosslinking agents used in the process of thisinvention are dicumyl peroxide,

6 t CH3-HC CH3-( J-CH3 (5113 Ha Most especially preferred for use inpreparing films to be used in the food packaging industry is2,5-dimethyl-2,5- di-(tert-butylperoxy) hexane.

The crosslinking agents can be used singly or in combination. It is onlynecessary that the gel point of the mixture be sufficiently high toenable shaping of the mixture in a tube extruder at temperatures abovethe melting point of the polyethylene resin base. Because of thislimitation some crosslinking agents, e.g. diacyl peroxides, cannot beused alone since they decompose too fast at extrusion temperatures.However, they can be used in small proportions (e.g. to of the totalamount of crosslinking agent) together with agents having much longerhalf-lives at extrusion temperatures. These small amounts will notsignificantly lower the gel point of the mixture and also are valuablein assisting fulfillment of the requirement of rapid curing of theshaped tube after it leaves the extruder to avoid distortion.

FIGURE 1 is a schematic illustration of an arrangement of apparatuselements suitable for performing the process of this invention in afully continuous manner.

As shown in FIGURE 1, and 21 are suitable storage hoppers forparticulate (granular or pelleted) normally solid polyethylene resin andfor crosslinking agent, respectively. As discussed hereinabove, thecrosslinking agent can be used in its available form (powder or liquid)or as a solution in an inert solvent. Polyethylene and crosslinkingagent are fed in appropriate proportions to blender 22 where they areformed into a substantially homogeneous mixture. The mixture can bestored if desired or immediately transferred to feed hopper 2 ofrotating screw extruder 1. The extiuder screw (not shown) is driven bypower source 24 which rotates drive shaft connected to the screw. Theextruder is heated in known manner to a temperature sufficient to flux(melt) the polyethylene but below temperatures at which the crosslinkingagent will decompose. The fluxed mass of polyethylene is pushed out ofthe extruder through adapter 3 to tubing die 4 which faces in avertically downward direction. Polyethylene tube T formed by the diefalls under the force of gravity through cooling ring 5 and into heatedzones 6A, 6B, and 6C situated immediately below the cooling ring. Thecooling ring prevents heat transfer from top heated zone 6A to the die4. Such heat transfer can result in undesirable premature crosslinkingof the tube in the die.

For better understanding heated zones 6A, 6B and 6C are shown in partialcross section. Each zone in the preferred embodiment consists of acylindrical heavy-duty furnace heated by electrical resistance wires(not shown). The furnaces can be wired so as to have individuallycontrolled heated zones or each furnace can be controlled as a unit.Inert gas such as nitrogen is fed through lines 28, 29 and 30 throughvalves 26 and 27, respectively, and thence to sparging rings 31 and 32,respectively, suitably located at the top and bottom of the contiguouslyconnected furnaces. Other means of maintaining an inert atmosphere inthe interior of the furnaces will be readily apparent.

The polyethylene tubing T is cured as it passes through the furnaces 6A,6B and EC to a percent gel greater than about 20% and preferably greaterthan about 50% by appropriate adjustments of temperature and residencetime in the furnace, as previously described.

As it leaves the furnace the cured tubing is cooled to a temperaturebelow that at which it adheres to itself. Cooling can be in any desiredmanner. In the preferred embodiment shown in FIGURE 1 the cured tube ispassed into a quench tank 7 containing a suitable inert liquid 8 (suchas water) which is maintained at temperatures below about centigrade forlow-density polyethylene tubing or below about 110 centigrade for highdensity polyethylene tubing. After quenching the tube is passed underroll 11 which may be a driven roll if desired, and thence tocounterrotating pinch rolls 12a and 1212. At this point of the process,the tube can be coiled up as a tape and stored or shipped as such ifdesired or preferred.

In the preferred embodiment of this invention, the tube is passedbetween rolls 12a and 12b to another pair of counterrotating rolls 13aand 1312 which are located in tank 9. An inert liquid 10 in tank 9 isused to reheat the tube to a temperature at which it can be expanded,e.g., between about and about centigrade for ,cured tubing composed oflow density polyethylene. In-

ert liquid 10 must necessarily have a boiling point great er than about110 to centigrade when the cured tubing is composed of medium or highdensity polyethylene. In such case, it is generally preferred to place afurnace, an oven, or an infra-red heater between paired rolls 13a-13band 14a-14b in lieu of tank 9 and liquid bath 10. The remainder of theapparatus illustrated, comprising rolls 14a and 14b; bubble 19 filledwith inert gas 16; rolls 15a and 15]); roll 17 and takeup device 18 issimilar to that used in production of other oriented films (see, e.g.,Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 14, page 751) andneed not be further described herein.

In the following examples apparatus such as that described immediatelyabove was used to continuously produce crosslinked polyethylene tubingunder various conditions.

EXAMPLES 2-24 In these examples the polyethylene used was commerciallyavailable low density (0.92) resin in pelleted form. The cross-linkingagent was 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane. The polyethylenepellets were blended with 0.75% by weight (based on weight of thepolyethylene) of the liquid peroxide for twenty minutes at roomtemperature in a Patterson-Kelley twin shell blender to form asubstantiallyhomogeneous mixture.

The mixture was fed into a one-inch rotating screw extruder having alength to diameter ratio of 20 to 1 and a compression ratio of 4 to 1.No breaker plate or screen pack was used. The polyethylene was extrudedthrough an adapter and out of a /4-inch by 0.040-inch tubing die in avertically downward direction from a platform 10 feet high upon whichthe extruder assembly was mounted. The three sections of the extruderbarrel were maintained at 220, 240 and 260 Fahrenheit, respectively,with lowest temperatures at the feed end. Measured die temperatures werewithin the range of from 260 to 310 Fahrenheit.

Directly below the die (about 2 inches) was a heated curing zoneconsisting of three Hevi-Duty resistance heated furnaces. Furnace 1(corresponding to zone 6A in FIGURE 1) was a 3000 watt furnace 26 /2inches long with a 3 /s-inch diameter heating chamber. There were threecontrolled heating zones in this furnace 3 /2 inches, 12 inches and 8 /2inches long, respectively. By turning case the tubing was fed to a waterfilled quench tank maintained at temperatures below about 70 centigrade.The furnace temperatures shown in the table are those measured in thetop zone of furnace 1, at the middle of furnace 1 and at the middle offurnace 2. As noted above, furnace 3 was used as an annealing chamberand thus was not heated.

Some of the runs shown in Table I lasted for more than about 15 hours.Reproducible results were obtained in each instance. The percent gelthroughout the cross section of tubing produced did not vary more than2%, showing the crosslinking was uniform. The cured tubing was readilyfabricated into tubular film by feeding through a hot water bath toreheat it to about 95 to 100 centigrade and expanding by the trappedbubble technique. The film produced was satisfactory for use in shrinkpackaging of various food.

Table I.C-n'tinu0u.r production- 09 cured polyethylene tubing HeatingCuring Zone 'Iemper- Tube Measurement atures, Degrees FahrenheitProduction Extrnder Extruder Rate, Feet Run No. Screw Output of CuredSpeed (grams per Tubing per Wall Lay Flat Percent (r.p.m.) minute) TopZone Middle Middle minute Thickness Width Gel (In Furnace 1 Furnace 1Furnace 2 (inch) (inch) Boiling Toluene) 30 30.0 850 550 500 21'2 0181/4 55. /53. 6 40 41.9 850 550 500 26'6" 019 3/16 49. 8/45. 2 30 31.0950 550 500 1811 018 1/4 54. 4/56. 7 40 42. 2 950 550 500 247 022 1/459. 0/59. 5 30 30.0 1,050 550 500 167 021 1/4 61. 2/54. 4 40 42. 5 1,050 550 500 21'1" 020 1/4 55. 6/54. 8 30 32.0 1, 150 550 500 8 020 l/450. 8/48. 5 40 42. 5 1, 150 550 500 195 022 1/4 51. 5/54. 6 29. 5 850550 550 1811 018 1/4 56. 5/56. 7 42. 0 950 550 550 22'2" 018 1/4 56.9/57. 7 30 31.0 1,050 550 550 154 019 5/16 51. 7/53. 4 40 40. 8 1,050550 550 19'1" 020 5/16 57. 6/56. 7 40 41. 5 1, 150 550 550 1611 022 5/1653.1/52. 5 30 30. 0 900 550 500 172 016 5/16 53. 4/54. 1 40 42. 0 900550 500 221 118 1/4 60. 0/56. 8 30 31. 0 925 550 500 169 016 5/16 53.5/53. 1 40 41. 5 925 550 500 22'3" 020 1/4 56. 6/58. 9 30 30. 8 950 550500 169 021 1/4 56. 6/58. 8 40 41. 8 950 550 500 21 1 018 1/4 59. 8/58.5 30 30.0 975 550 500 168 018 1/4 56. 6/57. 8 40 42. 0 975 550 500 20 9020 5/16 57. 9/57. 3 3O 29. 5 1, 000 550 500 15'7 018 l/4 53. 5/55. 3 4041. 0 1, 000 550 500 201 021 1/4 56. 1/56. 6

the furnace over, the top zone (closest to the die) could be either 3 /2or 8 /2 inches. In these examples an individually controlled 8 /z-inchtop zone in Furnace 1 was used. Furnace 2 (corresponding to zone 63 inFIGURE 1) was a 1900 watt furnace, 25 inches long with a 3 /2- inchdiameter heating chamber and a temperature control positioned at itscenter. Furnace 3 (corresponding to zone 6C) had a 2 -inch diameterchamber. Furnace 3 was used as an annealing chamber in this example sothat no heating power was supplied thereto. Sparging rings forintroducing nitrogen into the furnace chambers were provided at the topof furnace 1 and bottom of furnace 3. A small needle-like pipe in thedie was used to place a small amount of inert gas such as nitrogeninside the tube issuing from the extruder so as to prevent the tube fromcollapsing upon itself beforevit was cured. Another similar pipeadjacent to the first served as a vent for gaseous products formed bydecomposition of the crosslinking agent as the curing step proceeds.Means for monitoring the amount of inert gas and regulating the ventingof decomposition products so as to avoid expansion of the tube before itis cured will be readily apparent to those skilled in the art. To avoidover-heating of the die a 1 /s-inch thick cooling ring was positionedEXAMPLES 25-26 In these examples the polyethylene used was a high (.950)density resin having a melt index of 5.0 and in flake-like form. Asolution containing 30 grams of 2,5-dimethyl-2,5-di(tert-butylperoxy)-3-hexyne per 200 cubic centimeters ofpetroleum either was prepared. The resin and peroxide solution wereblended in a twin-shell blender in proportions to provide 0.75 percentby weight peroxide based on the weight of the polyethylene.

Portions of the. above blend were fed through the extruder used inExamples 2-24 above and out of a inch by 0.037 inch tubing die. Again noscreens or beaker plates were used. The three sections of the extruderbarrel were maintained at 260, 280 and 300 Fahrenheit respectively, withlowest temperatures at the feed end. Temperature measured at the die was320 Fahrenheit. The extruder screw was rotated at 40 revolutions perminute.

The shaped tube was fed vertically downward through a heated curing zoneessentially the same as that described in Examples 2-24 above, using an8 /2 inch top zone in furnace l. The cured tubing produced was quenchedin a 45 Fahrenheit water bath.

Data for these examples is shown in Table I. Furnace temperatures shownare those measured at the top of furnace 1, at the bottom of furnace 1,and at about the l 1 middle of furnace 2. Furnace 3 was used as anannealing chamber.

8. Process as defined in claim 7 wherein an inert gas atmosphere ismaintained in said heated curing zone.

Table II.-Cntinu0us production of cured polyethylene tubing HeatingCuring Zone Temperatures, Tube Measurement Ext-ruder Extruder DegreesFahrenheit Production Screw Output Rate, Feet Run No. Speed (grams perof Cured (r.p.m.) minute Tubing per Wall Lay Flat Percent Top ZoneMiddle Middle minute Thickness Width Gel (In Furnace 1 Furnace 1 Furnace2 (inch) (inch) Boiling Xylene) The cured high-density polyethylenetubing produced in these examples has excellent solvent resistance.Furthermore, because of its excellent resistance to distortion orbursting at high temperature and pressure, it is ideally suited for useas a hot water conduit.

The cured high density polyethylene tubing was expanded to form filmafter it was reheated to temperatures of from about 120 to about 135centigrade. The film so produced has very good clarity and sparkle.These properties have not been previously achieved in films produced byirradiation crosslinking of high density polyethylene.

Insofar as is known, no one has heretofore been able to produce unfilledchemically crosslinked polyethylene tubing at the commercial speedsshown in the Examples 2-26 (above) without some means for supporting thetube between the shaping and curing steps.

Thus it is seen that this invention provides a process and apparatus forthe continuous production of unfilled, unsupported chemicallycrosslinked polyethylene tubing and also for the continuous productionof shrinkable crosslinked polyethylene film having properties which makeit a very desirable material for use in various packaging industries,particularly in the food packaging industry.

What is claimed is:

1. Process for continuous production of shrinkable polyethylene filmwhich comprises forming a substantially homogeneous mixture of anormally solid polyethylene and from about 0.2 to about 4.0 percent byweight, based on the weight of the polyethylene, of a crosslinkingagent; shaping said mixture into a tube by extruding the same attemperatures between the melting point of the polyethylene and the gelpoint of the mixture; feeding the extruded tube into a heated curingzone maintained at temperatures sufficient to heat the mixture above thedecomposition temperature of the crosslinking agent; passing said tubethrough said curing zone at a rate sufi'icient to provide a residencetime in said zone at least equal to about three half-lives of saidcrosslinking agent; cooling the thus cured tubing to a temperature belowthat at which it adheres to itself; reheating the cured, cooled tubingto temperatures at which it can be expanded and expanding said reheatedtube to form tubular film therefrom.

2. Process as defined in claim 1 wherein said crosslinking agent is anorganic peroxygen compound.

3. Process as defined in claim 2 wherein said crosslinking agent is usedin an amount between about 0.5 percent and about 1.0 percent by weight.

' 4. Process as defined in claim 2 wherein said crosslinking agent isdicumyl peroxide.

5. Process as defined in claim 2 wherein said crosslinking agent is2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane.

6. Process as defined in claim 2 wherein said crosslinking agent is2,5-dimethyl-2,5-di-(tert-butylperoxy)-3-hexyne.

7. Process as defined in claim 1 wherein said tube is extruded in asubstantially vertically downward direction and is gravity fed to saidcuring zone.

9. Process as defined in claim 8 wherein said inert gas is a member ofthe group consisting of nitrogen and carbon dioxide.

10. Process for the continuous production of shrinkable polyethylenefilm which comprises forming a substantially homogeneous mixture of anormally solid polyethylene resin having a density of from about 0.910to about 0.925 with from about 0.5 to about 1.0 percent by weight of2,5-dimethyl-2,5-di (tert butylperoxy) hexane, shaping said mixture intoa tube by continuously extruding the same at temperatures between about240 Fahrenheit and about 310 Fahrenheit and in a substantiallyvertically downward direction; immediately gravity feeding said extrudedtube into a heated curing zone maintained at temperatures sufiicient toheat the shaped mixture to a temperature at which2,5-dimethyl-2,5-di-(tert-butylperoxy)- hexane has a half life of lessthan about 0.5 minute; con tinuously passing said tube through saidcuring zone at a rate sufiicient to provide a residence time at saidlastnamed temperature at least equal to three half-lives of said2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexane, whereby said tube iscured; continuously feeding said cured tube to a cooling zone where itis quenched to temperatures below about 70 centigrade; reheating saidcured, quenched tube to temperatures in the range of from about to aboutcentigrade; expanding said reheated tube to form tubular shrinkable filmtherefrom; and recovering said film.

11. Process for the continuous production of shrinkable polyethylenefilm which comprises forming a substantially homogeneous mixture of anormally solid polyethylene resin having a density of from about .950 toabout .980 with from about 0.5 to about 1.0 percent by weight of 2,5-dimethyl-2,5-di-(tart-butylperoxy)-3-hexyne, shaping said mixture into atube by continuously extruding the same at temperature between about 260Fahrenheit and about 330 Fahrenheit and in a substantially verticallydownward direction; immediately gravity feeding said extruded tube intoa heated curing zone maintained at temperatures sufiicient to heat theshaped mixture to a temperature at which 2,S-dimethyl-2,S-di-(tert-butylperoxy) -3-hexyne has a half life of less than about 0.5minute; continuously passing said tube though said curing zone at a ratesufficient to provide a residence time at said last-named temperature atleast equal to three half-lives of said2,5-dimethyl-2,5-di-(tert-butylperoxy)-3-hexyne, whereby said tube iscured; continuously feeding said cured tube to a cooling zone where itis quenched to temperatures below about 100 centigrade, reheating saidcured, quenched tube to temperatures in the range of from about 100 toabout centigrade; expanding said reheated tube to form tubularshrinkable film therefrom; and recovering said film.

12. Process for continuous production of unfilled, unsupportedchemically crosslinked polyethylene tubing which comprises forming asubstantially homogeneous mixture of a normally solid polyethylene andfrom about 0.2 to about 4.0 percent by weight, based on the weight ofthe polyethylene, of a crosslinking agent; shaping said mixture into atube by extruding the same at temperatures between the melting point ofthe polyethylene and the gel point of the mixture; feeding the extrudedtube into a heated curing zone maintained at temperatures sufiicient toheat the mixture above the decomposition temperature of the crosslinkingagent; passing said tube through said curing zone at a rate suflicientto provide a residence time in said zone at least equal to about threehalf-lives of said crosslinking agent; and cooling the thus cured tubingto a temperature below that at which it adheres to itself. I

13. Process as defined in claim 12 wherein said crosslinking agent is anorganic peroxygen compound.

14. Process as defined in claim 13 wherein said crosslinking agent isused in an amount between about 0.5 percent and about 1.0 percent byweight.

15. Process as defined in claim 13 wherein said crosslinking agent isdicumyl peroxide.

16. Process as defined in claim 13 wherein said crosslinking agent is2,5-dimethyl-2,5-di-(tert-butylperoxy) hexane.

17. Process as defined in claim 13 wherein said crosslinking agent is2,5-dimethyl-2,5-di-(tert-butylperoxy)-3- hexyne.

18. Process as defined in claim 12 wherein said tube is extruded in asubstantially vertically downward direction and is gravity fed to saidcuring zone.

19. Process as defined in claim 18 wherein an inert gas atmosphere is'maintained in said heated curing zone.

20. Process for continuous production of unfilled, unsupportedchemically crosslinked polyethylene tubing which comprises forming asubstantially homogeneous mixture of a normally solid polyethylene resinhaving a density of from about 0.910 to about 0.925 with from about 0.5to about 1.0 percent by weight of 2,5-dimethy1- 2,5-di-(tert-butylperoxy)-hexane, shaping said mixture into a tube bycontinuously extruding the same at temperatures between about 240Fahrenheit and about 310 Fahrenheit and in a substantially verticallydownward direction; immediately gravity feeding said extruded tube intoa heated curing zone maintained at temperatures sufficient to heat theshaped mixture to a temperature at which 2,5dimethyl-2,5-di-(tert-butylperoxy)-hexane has a half-life of less thanabout 0.5 minutes; continuously passing said tube through said curingzone at a rate suflicient to provide a residence time at said last-namedtemperature at least equal to three half-lives of said 2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexane, whereby said tube is cured; andcontinuously feeding said cured tube to a cooling zone where it isquenched to temperatures below about 70 centigrade.

21. Process for continuous production of unfilled, unsupportedchemically crosslinked polyethylene tubing which comprises forming asubstantially homogeneous mixture of a normally solid polyethylene resinhaving a density of from about .950 to about .980 with from about 0.5 toabout 1.0 percent by Weight of 2,5-dimethyl-2,5-di- (tert-butylperoxy)-3-hexyne, shaping said mixture into a 15 tube by continuously extrudingthe same at temperature between about 260 Fahrenheit and about 330Fahrenheit and in a substanitally vertically downward direction;immediately gravity feeding said extruded tube into a heated curing zonemaintained at temperatures sufficient to heat the shaped mixture to atemperature at which 2,5- dimethyl-2,5-di-(tert-butylperoxy)-3-hexynehas a half life of less than about 0.5 minutes; continuously passingsaid tube through said curing zone at a rate sufiicient to provide aresidence time at said last-named temperature at least equal to threehalf-lives of said 2,5-dimethyl-2,5-di- (tert-butylperoxy)-3-hexyne,whereby said tube is cured; and continuously feeding said cured tube toa cooling zone where it is quenched to temperatures below aboutcentigrade.

References Cited by the Examiner UNITED STATES PATENTS 2,919,474 1/60Cole 264-211 2,979,777 4/61 Goldman 18-67 3,008,186 11/61 Voigt 18-143,009,208 11/61 Pirot 18-67 3,022,543 2/ 62 Baird et al. 264-2093,044,114 7/62 Pirot 18-14 ROBERT F. WHITE, Primary Examiner.

MICHAEL V. BRINDISI, ALEXANDER H. BROD- MERKEL, Examiners.

1. PROCESS FOR CONTINUOUS PRODUCTION OF SHRINKABLE POLYETHYLENE FILMWHICH COMPRISES FORMING A SUBSTANTIALLY HOMOGENEOUS MIXTURE OF ANORMALLY SOLID POLYETHYLENE AND FROM ABOUT 0.2 TO ABOUT 4.0 PERCENT BYWEIGHT, BASED ON THE WEIGHT OF THE POLYETHYLENE, OF A CROSSLINKINGAGENT; SHAPING SAID MIXTURE INTO A TUBE BY EXTRUDING THE SAME ATTEMPERATURES BETWEEN THE MELTING POINT OF THE POLYETHYLENE AND THE GELPOINT OF THE MIXTURE; FEEDING THE EXTRUDED TUBE INTO A HEATED CURINGZONE MAINTAINED AT TEMPERATURES SUFFICIENT TO HEAT THE MIXTURE ABOVE THEDECOMPOSITION TEMPERATURE OF THE CROSSLINKING AGENT; PASSING SAID TUBETHROUGH SAID CURING ZONE AT A RATE SUFFICIENT TO PROVIDE A RESIDENCETIME IN SAID ZONE AT LEAST EQUAL TO ABOUT THREE HALF-LIVES OF SAIDCROSSLINKING AGENT; COOLING THE THUS CURED TUBING TO A TEMPERATURE BELOWTHAT AT WHICH IT ADHERES TO ITSELF; REHEATING THE CURED, COOLED TUBINGTO TEMPERATURES AT WHICH IT CAN BE EXPANDED AND EXPANDING SAID REHEATEDUBE TO FORM TUBULAR FILM THEREFROM.